1. Field of the Invention
The present invention is directed to a process for crosslinking a thermoplastic polymer and to a crosslinking system, a cyclic peroxide and co-agent therefor, used therein. More specifically, the present invention is directed to a process of crosslinking a thermoplastic polymer by contacting it with a crosslinking system which includes a cyclic seven-membered peroxide compound and a crosslink-promoting polyfunctional ethylenically unsaturated compound.
2. Background of the Prior Art
The utilization of compounds having peroxy functionality to crosslink thermoplastic polymers is well established in the art. That there are so many peroxide compounds utilized in this application gives evidence of the variety of thermoplastic polymers subject to crosslinkage.
Independent of the identity of the thermoplastic polymer subject to crosslinkage there are three properties commonly sought in all peroxide compounds employed in this use. These properties are, indeed, independent of the peroxide compound's suitability as a crosslinking agent. The first of these is shelf-life stability. This is a threshold requirement. It would not do well to utilize a peroxide having excellent crosslinking properties which degrades prior to use.
The second of these properties is ease of synthesis. Obviously, the easier the synthesis, as measured by the number of steps required to produce the peroxide, the yield of each step, the level of difficulty associated with separating the product from the reaction mixture and the like, the lower will be the cost of the peroxide.
Finally, the third desired property, directly related to the specific function to which the peroxide is put, thermoplastic polymer crosslinkage, is onset temperature. Onset temperature is the temperature at which uncontrolled decomposition begins. The onset temperature of a peroxide must be such that at the temperature at which the thermoplastic polymer is to be crosslinked, the peroxide undergoes decomposition at a rate sufficient to effect such crosslinking in a reasonable period of time.
A class of peroxide compounds that meets the desired criteria, i.e. of possessing the three aforementioned properties, are 1,2,4-trioxacycloheptanes in which three oxygen atoms are contained in a seven-membered ring. Specific examples of these cyclic peroxide compounds are known in the art. U.S. Pat. Nos. 4,956,416; 5,272,219; 5,360,867; 5,399,630; and 5,457,162 each describe a seven-membered cyclic monoperoxy compound useful in curing unsaturated polyester resins, polymerizing ethylenically unsaturated monomers, modifying molecular weight and molecular weight distribution of polypropylenes, grafting monomers onto polymers, and crosslinking polymers and elastomers and the like. It is emphasized that although a simple recitation is made in these references stating that the peroxides of these patents may be employed in a process of crosslinking polymers, there is no suggestion of combining a seven-membered cyclic peroxide compound with other compounds for enhancement of their crosslinking properties or any other of the uses alleged for these compounds.
In further regard to seven-membered cyclic peroxide ring compounds, a survey article, directed to organic peroxide compounds in general, which appears in the "Encyclopedia of Chemical Technology", Fourth Edition, Vol. 18, pages 230-310 (1996), describes 1,2,4-trioxacycloalkanes at pages 259-260. Although such compounds are set forth in the aforementioned article, no specific use for these compounds is provided.
In addition to seven-membered cyclic compounds having peroxy functionality, other classes of peroxide compounds are known which-are related to the present invention. One such class of compounds is the class known as the monoperoxyacetals, described in U.S. Pat. No. 2,776,319. This class of acyclic compounds is said to be useful as polymerization catalysts, insecticides, fungicides, bactericides and defoliants.
Another class of peroxide compounds of interest is "ether peroxides." Compounds of this class are described in U.S. Pat. Nos. 3,576,826 and 3,822,317. Ether peroxides of the type included in these patents are recited to have utility as styrene polymerization initiators.
Another class of acyclic peroxide compounds are bis-monoperoxyacetals and bis-monoperoxyketals. These compounds are of interest because of their utility as polymer crosslinking agents. This class of compounds is the subject of U.S. Pat. No. 3,956,398.
A similar class of monoperoxyketals, containing a cyclohexane ring, is disclosed in U.S. Pat. No. 5,004,780. The cyclic monoperoxyketals of this patent are stated to be useful as polymerization initiators and curing agents for unsaturated polyester resins.
Classes of hydroxyperoxides useful in curing of unsaturated polyester resin compositions, in free radical polymerization of ethylenically unsaturated monomers, in curing of elastomers, in crosslinking thermoplastic polymers, in modifying polypropylene homo- and co-polymers and the like are set forth in U.S. Pat. Nos. 5,304,649 and 5,387,654. These compounds are all characterized by possessing both hydroxy and monoperoxyketal functionality.
The above extended discussion of peroxide compounds emphasizes that although a plethora of peroxide compounds are employed in polymerization processes, there is still a present and continuing need in the art for peroxide compounds to serve as initiators in the high temperature crosslinkage of thermoplastic materials.